Propylene is an important commodity petrochemical useful in a variety of processes for making plastics and other chemical compounds. For example, propylene is used to make various polypropylene plastics, and in making other chemicals such as acrylonitrile and propylene oxide.
The petrochemical industry has known for some time that oxygenates, especially alcohols, are convertible into light olefins, such as propylene. The preferred conversion process is generally referred to as an oxygenate-to-olefin (OTO) or specifically as a methanol-to-olefins (MTO) process, where methanol is converted to primarily ethylene and/or propylene in the presence of a molecular sieve catalyst.
Various byproducts are produced in the OTO reaction process. Some of these byproducts should be separated from the propylene product in order to provide propylene suitable for polymerization disposition. These byproducts may include components that are heavier than propane and propylene, such as C4+ components (olefinic and aliphatic) as well as multiply unsaturated components such as acetylene, methyl acetylene and propadiene.
Additionally, oxygenate compounds such as alcohols, aldehydes, ketones, esters, acids and ethers (particularly dimethyl ether “DME”) in the C1 to C6 range as well as trace quantities of aromatic compounds may be formed in OTO reactors or in OTO effluent processing. A small amount of oxygenate from the feedstock, e.g., methanol and/or DME, can pass through the OTO reactor with the product effluent without being converted to the desired product. As a result of oxygenate synthesis and/or incomplete oxygenate conversion in an OTO reactor system, an effluent from an OTO reactor can contain undesirably high concentrations of oxygenate compounds. These oxygenates, particularly light oxygenates, are in amounts that would make the propylene off-specification for its preferred disposition, e.g., polymerization.
Conventional propylene production facilities that produce propylene for polymerization disposition are required by the industry to produce very pure propylene. Conventional polymerization grade propylene contains at least 99.5 weight percent propylene, with the balance being mostly propane. A minor amount of other contaminants such as hydrogen, oxygen, and water, typically on a wppm level, may be tolerated in polymerization grade propylene. The high purity requirements in the industry are directly related to the usage of high activity catalysts for the formation of polypropylene. For example, bulky ligand metallocene-type catalyst systems such as those described in, for example, U.S. Pat. No. 5,324,800, are highly sensitive to oxygen, ethers, ketones, aldehydes, carbon dioxide, and other contaminants.
As a result of the high purity propylene requirements, various processing schemes have been developed for separating one or more contaminants from propylene-containing effluent streams. For example, U.S. Pat. No. 6,121,503 to Janssen et al., the entirety of which is incorporated herein by reference, discloses a process for converting an oxygenate feed to high purity olefins such as polymer-grade propylene.
The equipment count and resources necessary for processing crude propylene product streams and for providing high purity polymerization grade propylene can substantially increase both investment and operating costs. Thus, the need exists for the ability to polymerize propylene derived from a propylene-containing stream that also contains a certain level of impurities, e.g., byproducts of the OTO reaction process.